This invention relates to an optical waveguide modulator configuration, particularly, an optical waveguide modulator configuration preferably applied to waveguide type optical intensity-modulators, phase-modulators, and polarization scramblers employed in high speed and large capacity optical fiber-communication systems and wavelength division multiplexing systems.
With the recent advances in high speed and large capacity optical fiber-communication systems, from the viewpoint of broad bandwidth, low chirp and low propagation loss characteristics, waveguide type external modulators using substrates made of lithium niobate (LiNbO3: hereinafter often abbreviated to xe2x80x9cLNxe2x80x9d) are being realized, rather than conventional diodes which are direct-modulation type.
FIG. 1 is a cross sectional view showing an example of a conventional optical waveguide modulator.
An optical waveguide modulator 10, as shown in FIG. 1, has a substrate 1 made of xe2x80x9cLNxe2x80x9d etc., a Mach-Zehnder type interferometer 2, formed by thermal diffusion of Ti into the substrat 1, a travelling wave-type signal electrode 3 and ground electrodes 4 made of Au that is applied directly on the optical waveguide 2, or on a nearby surface.
Moreover, for lowering the absorption loss of the lightwave travelling in the optical waveguide 2 by the travelling wave-type signal electrode 3 and the ground electrodes 4 and matching of velocity between the lightwave and microwave travelling on the signal electrode 3, a buffer layer 5 made of silicon dioxide (SiO2) is formed between the substrate 1 and the signal electrode 3 and the ground electrode 4.
Furthermore, with the developments in recent optical communication systems, multi-functions as well as high speeds and large capacity are required. In particular, the wavelength-multiplexing in the same optical waveguide, the switching and the exchanging of optical transmission guides are sought. Such communication systems are being realized with a wavelength division multiplexing method (hereinafter often abbreviated to xe2x80x9cWDM systemxe2x80x9d) using an optical fiber amplifier (hereinafter often abbreviated to xe2x80x9cEDFAxe2x80x9d).
The WDM system transmit, by a single optical fiber, multiple lightwaves having different wavelengths from the corresponding optical sources, to each lightwave being modulated by one of the different signals. That is, the system requires to prepare multiple optical modulators each connected with the corresponding optical source, and any one of the signals modulated by the multiple optical modulators is transmitted by a single optical fiber. The EDFA is provided in its transmission guide to amplify the gain of transmitted lightwave.
The WDM system enables the transmission capacity of the whole communication system to be increased without augmenting the number of optical fibers and the bit rate of each signal.
The WDM system requires the transmission condition of each lightwave to be constant. However, there is a problem that received intensity of an optical signal at the detector sometimes fluctuate in each transmitted lightwaves, on account of the wavelength dependency of the EDFA""s gain and the change of the output power with time from each optical source, etc.
To overcome this problem, the integration of an attenuator with each of the optical modulator is being attempted. FIG. 2 is a top plan view showing an example of a conventional optical waveguide modulator to which an attenuator is integrated. FIGS. 3(a) and 3(b) are cross sectional views of the optical modulator shown in FIG. 2. FIG. 3a is a cross sectional view of an optical modulation part, taken on line A-A"" of FIG. 2, and FIG. 3b is a cross sectional view of an attenuator part, taken on line B-B"" of FIG. 2.
A conventional optical waveguide modulator 30 shown in FIG. 2 and 3 has a substrate 11 made of a material having an electrooptic effect, a first interferometer 12 and a second interferometer 13 formed by thermal diffusion of Ti into the substrate. Then, it has a buffer layer 14 made of silicon dioxide, etc. formed on the substrate 11. On the buffer layer 14 are formed a first signal electrode 15, first ground electrodes 16, a second signal electrode 17 and second ground electrodes 18.
Electrical inputs of the first and the second signal electrodes 15 and 16, are connected with external electric power supplies 21 and 22, respectively, the output of the first signal electrode 15 being terminated via a resistor xe2x80x9cRxe2x80x9d and a capacitor xe2x80x9cCxe2x80x9d. Metal-cladding type waveguide polarizers 23 and 24 are provided in the input and output sides of the optical modulator 30.
The first interferometer 12, the first signal electrode 15 and the first ground electrodes 16 constitute an optical modulation part 28. The second optical waveguide 13, the second signal electrode 17 and the second ground electrodes 18 constitute an attenuator part 29. The first signal electrode 15 and the first ground electrodes 16 constitute an electrode for modulation. The second signal electrode 17 and the second ground electrodes 18 constitute an electrode for attenuation. And, the first interferometer 12 is in series connected with the second interferometer 13 in the boundary xe2x80x9cHxe2x80x9d between the optical modulation part 28 and the attenuator part 29. The arrow in FIG. 2 depicts a travelling direction of a lightwave.
The buffer layer 14 is formed to prevent the absorption of the lightwave guiding in the optical waveguide by the modulation electrode and the attenuator electrode.
When a lightwave having a wavelength of xcex1 is incident into the optical waveguide modulator 30, it is on-off switched and thereafter its intensity is controlled in attenuator part 29. That is, by compulsive attenuation of the intensities of specific optical signals having large output powers, the intensity of each optical signal having different wavelengths, is equalized in the whole communication system.
Such an optical waveguide modulator, as shown in FIG. 1, is desired to be enhanced in modulation efficiency in view of reducing the load for a high frequency driver. Thus, the distance between the optical waveguide and the travelling type signal electrode and electrode gap are required to be shorter and narrower, respectively, to lower the driving voltage of the optical modulator.
However, as shown in the optical waveguide modulator 10 in FIG. 1, when the buffer layer 5 is formed between the substrate 1 and the travelling type signal electrode 3 or the like, the distance between the optical waveguide 2 and the signal electrode 3 is inevitably increased and thereby the driving voltage can not be efficiently lowered.
Moreover, such an optical waveguide modulator as in FIGS. 2 and 3, is required to have relatively longer interaction length in optical modulation part 28 to realize low driving voltage. However, in the optical waveguide modulator having above-mentioned configuration, the attenuator part 29 can not have sufficient length because of limitation in wafer size. As a result, attenuator part 29 requires a very high driving voltage.
If the driving voltage is being higher, an electric discharge sometimes occur in the electrodes of the attenuator part 29, resulting in the destruction of the optical waveguide modulator 30 itself. Thus, the above optical modulator does not have a sufficient reliability.
In addition, if the driving voltage is being higher, there is practical problem that a DC drift due to the buffer layer 14 tends to be larger.
It is an object of the present invention to provide a new optical waveguide modulator configuration capable of reducing driving voltage in an optical modulation part or an attenuator part.
The first optical waveguide modulator applying the present invention has a substrate made of a material having an electrooptic effect, an optical waveguide to guide a lightwave, travelling wave-type signal electrode, ground electrodes and a buffer layer between the substrate and the above travelling wave-type electrodes. The buffer layer is formed only under the travelling wave-type signal electrodes so as to have a larger width than that of the travelling wave-type signal electrode and, at least a part of the buffer layer is embedded in a superficial layer of the substrate.
As above-mentioned, the conventional optical waveguide modulator 10, as shown in FIG. 1, has the buffer layer 5 on the entire main surface 1a of the substrate 1. However, there is a problem that the affection of the buffer layer under the signal electrode on the velocity matching between the lightwave in the optical waveguide and the microwave travelling in the signal electrode is not examined in detail.
From the standpoint of above-mentioned problem, present inventors examined about the buffer layer structure in detail.
As a result, they found the following fact:
The impedance matching of the electrodes and the velocity matching between the lightwave and the microwave are dominantly influenced by the part of the buffer layer under the travelling wave-type signal electrode and its nearby part, not so the part of the buffer layer under the ground electrodes and their nearby parts. It is also clarified that the driving voltage of the modulator is also influenced by the width of the buffer layer under the travelling wave-type signal electrode and its nearby part.
Moreover, the present inventors also found that the driving voltage depends on, surprisingly, whether the part of the buffer layer under the travelling wave-type signal electrode and its nearby part is embedded in the superficial layer of the substrate or not, and its embedded depth.
That is, the formation of the buffer layer having a larger width than that of the travelling wave-type signal electrode only under the signal electrode and its nearby part enables the driving voltage of the modulator to be reduced and the embedding of at least a part of the buffer layer into the superficial layer of the substrate enables the driving voltage to be reduced.
The first optical waveguide modulator according to the present invention was invented on the basis of the above facts obtained from extensive research by present inventors.
According to the modulator configuration by this invention, the absorption loss of the lightwave due to the electrodes can be reduced and the velocity matching between the lightwave and the microwave be achieved. In addition, it was found that it can reduce the driving voltage of the modulator and thereby the optical waveguide modulator having an improved modulation efficiency can be obtained.
Furthermore, the buffer layer may expect to be contaminated with impurity such as iron or sodium, in its fabrication process or absorb moisture with time. Thus, the formation of the buffer layer only under the travelling wave-type signal electrode and its nearby part according to the present invention, enables the absolute amount of impurities and the absorbed moisture to be reduced. As a result, these additional effects can prevent the fluctuation of the modulator characteristics and the increase of propagation loss of microwave due to the absorbed moisture in the buffer layer.
Herein, the wording xe2x80x9cthe width of the travelling wave-type signal electrodexe2x80x9d means the width of the face contacting with the buffer layer of the travelling wave-type signal electrode.
On the other hand, a second optical waveguide modulator, of the present invention, has an optical modulation part including a substrate made of a material having an electrooptic effect, a first Mach-Zehnder type interferometer formed on the substrate and an electrode for modulating, and an attenuator part including the substrate, a second Mach-Zehnder type interferometer in series connected with the first interferometer and an electrode for attenuating. Moreover, a buffer layer is formed on the. substrate, the thickness of the buffer layer in the attenuator part being thinner than that in the optical modulation part.
The present inventors have intensively studied to reduce the driving voltage of the attenuator part and found the following facts:
FIG. 4 is a graph showing the relation, found by the inventors, between the thickness xe2x80x9cTxe2x80x9d of the buffer layer in the attenuator part and the half-wavelength voltage xe2x80x9cVxcfx80xe2x80x9d as the driving voltage. As is apparent from the graph, surprisingly, the half-wavelength voltage xe2x80x9cVxcfx80xe2x80x9d decreases almost linearly without exhibiting its minimum value as the thickness of the buffer layer decreases.
In the case that the buffer layer is not formed on the substrate area having the attenuator part, the optical absorption of the attenuator electrode is very small.
The second optical waveguide modulator according to the present invention is derived from the basis of the above findings.
According to the second optical waveguide modulator of the present invention, since the thickness of the buffer layer in the attenuator part is thinner than that of the optical modulation part, the driving voltage of the attenuator part can be decreased. As a result, the electric discharge in the attenuator can be prevented. Moreover, in the case of not forming the buffer layer, DC drift, due to the buffer layer, can be inhibited. As a result, the optical waveguide modulator which has enough reliability for practical use can be provided.